1. Prepare a five to six-cup log mold. In a food processor, process biscuits and margarine together into fine crumbs. Set aside.
2. Puree chopped mango with a quarter cup sugar and two tablespoons water in a blender until smooth. You should have one cup of mango puree.
3. In a small saucepan over low heat, bring the puree and heavy cream to a simmer. Keep warm as you proceed to the next step.
4. In another saucepan over low heat, stir remaining sugar, water, and the corn syrup until the sugar dissolves.
5. Increase heat to medium–high and boil without stirring. You can swirl the pan as it boils and brush down the sides with a wet pastry brush to dissolve any crystals. Cook for eight minutes or until caramel is golden brown. Watch it carefully, since caramel goes from pale amber to golden brown very rapidly.
6. Remove pan from heat and slowly whisk in the warm mango mixture and a pinch of salt. The caramel will bubble vigorously, so make sure the mango mixture is warm to avoid scorching yourself.
7. Set aside three quarters of a cup of the mango-caramel sauce and allow to cool to room temperature. Cover and refrigerate the remaining mango-caramel sauce until ready for serving.
8. Soften the vanilla ice cream. Gently fold half the chopped macadamias into the ice cream.
9. Spoon half of the ice cream into the log cavity and smooth top.
10. Drizzle half of the room-temperature mango-caramel sauce over the ice cream. Using a fork, gently swirl it into ice cream.
11. Sprinkle chopped macadamias over the ice cream layer. Freeze till ice cream sets.
12. Repeat process with the remaining ice cream and caramel. Once ice cream is set, drizzle a layer of crumbs over the ice cream.
13. To serve, stir the reserved mango-caramel sauce to loosen and drizzle some over the cake. Sprinkle caramelized macadamias over the cake. You can also serve it with thinly sliced fresh mango.